Solid-state, non-contacting thermistor electronic switch

ABSTRACT

The switch includes a compressible plunger activated by depressing a key for permitting air to flow through a chamber housing a thermistor. The air flow changes the voltage drop across the thermistor for producing an output voltage signal representing the depressed key. A spring mechanism forces the key to return to its original position when released.

United States Patent Sullivan 1 1 Mar. 27, 1973 [5 1 SOLID-STATE, NON-CONTACTING 3,409,709 11/1968 Cleereman et al. ..323/68 THERMISTOR ELECTRONIC SWITCH 3,328,677 6 1967 Naegele ..323/68 3,597,676 8/197] Moore ..73/342 [75] Inventor: Norman F. Sullivan, Anahe1m, Cal1f.

[73] Assignee: North American Rockwell Corpora- Primary i ere man J. Hohauser ti Att0rney--L. Lee Humphries et al.

[22] Filed: Apr. 5, 1972 [57] ABSTRACT [21] Appl' 241282 The switch includes a compressible plunger activated by depressing a key for permitting air to flow through [52] US. Cl. ..307/ll7 a chamber housing a thermistor. The air flow changes [51] Int. Cl. ..H0lh 35/00 the voltage drop across the thermistor for producing [58] Field of Search..73/362 AR, 342, 349; 340/228; an output voltage signal representing the depressed 200.0 (TYPICAL) R. (5009 TYPICAL) key. A spring mechanism forces the key to return to its original position when released.

5 Claims, 2 Drawing Figures UTPUT .ZZpJd (TYPICAL) FIG. I

SOLID-STATE, NON-CONTACTING Tl-IERMISTOR ELECTRONIC SWITCH BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a solid-state, non-contacting electronic switching device and more particularly to such a switch using air flow across a housed thermistor for indicating the depression of a key.

2. Description of Prior Art Contacting switches are usually fabricated with moving and/or mechanical parts such as electrical switches, key switches and electronic keyboards. The mechanical parts are subject tocontact wear, contamination by foreign matter and exhibit low life and are relatively expensive to fabricate.

Non-contacting switches, called solid-state switches, include Hall-effect and optical-electronic devices. Preferably, the switch should have zero contact bounce, be free from foreign matter contamination with relatively few moving parts and an inexpensive fabrication process. The present invention, using a unique thermistor arrangement, provides the desired characteristics.

Thermistors in the self-heat state are thermally sensitive in that the thermistor resistance can be. changed as a function of any condition changing the rate at which heat is conducted away from it. For example, US. Pat.

No. 2,928,037 to Lawrence teaches a thermistor used as a liquid sensor device. In Lawrence, the thermistor is embedded in a probe assembly within a fuel tank. The fuel, above a certain level, surrounds the thermistor bead and maintains it at. a relatively low temperature. As the fuel level decreases, the thermistor bead is exposed to ambient atmosphere and the temperature thereof increases producing a drop in the resistance.

The present invention is distinguished from Lawrence in that Lawrence does not'teach or show an electronic switching device comprising a compressible plunger arrangement activated by depressing a key for permitting air to flow in a chamber housing a thermistor, which air flow changes the voltage drop across the thermistor for producing an output voltage signal representing the depressed key. A spring mechanism forces the key to return to its original position when released.

Nor does Lawrence teach or show a solid-state, noncontacting switching device which is sensitive to touch pressure or a device wherein the design of the diaphragm or air chamber will determine the output wave shape of the signal from the switch which is sensitive to touch pressure.

BRIEF SUMMARY OF THE INVENTION Briefly, the invention comprises a thermistor positioned inside of a compressible plunger arrangement. Air is forced through a narrow orifice over a small bead thermistor by compressing the plunger causing controlled cooling. This momentary cooling produces a voltage drop across the thermistor output terminals which is sensed and amplified with conventional circuitry. When the plunger is released, the compression spring returns the key to its original position. A small orifice in the lower portion of the plunger assembly allows ambient air to fill the plunger chamber for the next cycle. The current flowing through the thermistor the air chamber when depressed. Therefore, the designof the diaphragm or plunger arrangement utilized will determine the output wave shape of the signal from the switch when depressed. This touch feature of the switch can be easily adapted to keyboards for electronic pianos and organs.

Therefore it is an object of this invention to provide a switch using a thermistor which is responsive to air flow to generate an output signal indicative of a condition represented by the air flow.

It is another object of this invention to provide an inexpensive switching device capable of rapidly sensing and indicating controlled changes in the rate of heat removal about a thermistor.

Another object of this invention is to provide a noncontacting electronic switching device substantially free of moving parts, contact wear and foreign matter contamination.

Still another object of this invention is toprovide a non-contacting switching device capable of wave shaping its voltage change or output. I

A still further object of this invention is to provide a solid-state, non-contacting switching device which is sensitive to touch pressure and is readily adaptable in keyboards for electronic pianos and organs.

Still another object of this invention is to provide a thermistor switch which is relatively inexpensive to fabricate.

Other objects and advantages of this invention will be apparent from the description of the drawing, a brief description of which follows.

BRIEF DESCRIPTION OF DRAWING FIG. 1 shows a cross-sectional view of one embodiment of a switch assembly.

FIG. 2 shows the switch assembly as used in an electrical circuit.

DESCRIPTION OF PREFERRED EMBODIMENT FIG. 1 shows switch assembly 10 comprising base member 34 which is secured to a support (not shown). The support might be the floor of a keyboard housing. The base member 34, which may be comprised of aluminum, includes a centrally disposed rim 32 with a recessed area 30 for accommodating hollow cylinder 28. The interior (hollow) part of the cylinder 24, identified by numeral 9, provides an air chamber for the thermistor l8.

The thermistor is supported on a pedestal 22 on top of hollow cylinder 24 which includes a central channel 8 for accommodating electrical connector 20 which connects to the thermistor lead wires 5 and 6. The electrical connector passes through opening 7 in the base member 30 and channel 8 to electrically connect the thermistor 18 with external circuitry (not shown). The plunger 12 is matched to fit over the upper portion of the hollow cylinder 28 which has a narrow orifice 16 through its center extending over the thermistor 18. The spring 14 rests on the top of the hollow cylinder 28 and holds the plunger in its initial position and forces the plunger 12 to return to its original position when the key is released. A small hole in the side of the switch assembly 10 allows ambient air to enter the plunger chamber 24 identified by numeral 9, when the plunger 12 returns to its original position.

in operation, plunger 12 is actuated by depressing key 31 attached to plunger 12. Air in the inner cavity of plunger 12 is forced through narrow orifice 16 into the inside cylinder 24 across thermistor 18. The change in the air flow changes the voltage drop across the thermistor for producing an output voltage signal indicating the depression of the key.

The signal from the thermistor output leads and 6 can be used to drive IC logic directly. For example, as

shown in FIG. 2, a typicalbead thermistor with short response time can be operative in the self-heat mode by varying the current through R with 3 to' l0ma. of current decreasing T to approximately 100 ohms resistance. Upon depressing the switch for T cooling of the thermistor will cause a 200 to 3.00 mv. signal change to pass through C,. The signal may be amplified or used to drive low level logic circuits directly. When the key is released, spring 14 forces the key to its original position. A small orifice in the side of the plunger assembly 24 permitted the ambient air to rush into the plunger chamber 24 for the next cycle. The current through the thermistor 18 is returned to its established selfheat condition within 50 microseconds. The structure described above for controlling air across thermistors can be used for various switch applications. Where higher power applications require larger thermistors, cooling air may be utilized.

An alternate embodiment of this invention is shown where the response time of the thermistor switch varies as a function of the size and type of thermistor used for the switch and design of the air chamber 24 used to cycle the thermistor. in this example, the thermistor 18 in the switch assembly had a 0.007 in. diameter bead thermistor. The thermistor 18 was momentarily cooled by depressing the key and immediately releasing it. The key actuates the plunger causing air to flow through a narrow orifice 16, 0.030 in. diameter, into the plunger chamber 24 supporting the thermistor 18. The compression spring 14 in the plunger arrangement returned the key to its original position. A small orifice in the lower side of the plunger assembly permitted the ambient air to rush into the plunger chamber 22. The thermistor 18 was returned to its self-heat state within microseconds. The response time of the thermistor 18 was so'fast that its voltage output followed the exact thermal or cooling effect of the air chamber when depressed. Therefore, the design of the air chamber or the size of the thermistor will determine the output wave shape of the signal from the switch which is sensitive to touch pressure. The touch feature of this solidstate key switching device lends itself readily to adoption in keyboards for electronic pianos and organs. For example, when hitting the keyboard hard, the output will be hi hand when the key is touched lightl the output wi be low. Another feature of this soli -state key switching concept is the signal-to-noise or dynamic range. Tests of the dynamic range and signal-to noise for this thermistor switch indicate it is in excess of 40 decibels. This design parameter also contributes to its usefulness in applications such as electronic piano keyboards. This feature also enhances the thermistor switch usefulness for keyboards or other switch applications where signal-to-noise or dynamic .range requirements are critical.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principals involved, it is to be understood that the invention may be otherwise embodied without departing from the spirit and scope of the invention as hereinafter defined in the appended 3. The device recited in claim 1 including a chamber wherein said thermistor is positioned in the chamber, said chamber including air flow channels, said thermistor being mounted in the path of the air flow.

4. The device recited in claim 1 wherein said air flow providing means including a chamber for said thermistor having upper and lower openings for accommodating said air flow, and said plunger including means for forcing air through the upper opening and said air flow exiting out the lower opening of said chamber, and r a spring mechanism forcing the plunger to its original position when released and actuating air flow into said chamber prior to a subsequent cycle.

5. The devicerecited in claim 1 includinga chamber for said thermistor, the size of said chamber and said thermistor being determined as a function of the desired wave shape of the output signal for said thermistor.

s 1- a r 

1. A solid-state key switching device including a key actuated plunger comprising, a thermistor, a supporting assembly for said thermistor, said assembly including means providing air flow across said thermistor in response to the actuation of said plunger, said thermistor being responsive to said air flow for generating an output signal indicating a condition represented by the actuation of said plunger.
 2. The device recited in claim 1 wherein said supportiNg assembly comprises an enclosure for said thermistor having narrow openings for permitting air to flow across said thermistor.
 3. The device recited in claim 1 including a chamber wherein said thermistor is positioned in the chamber, said chamber including air flow channels, said thermistor being mounted in the path of the air flow.
 4. The device recited in claim 1 wherein said air flow providing means including a chamber for said thermistor having upper and lower openings for accommodating said air flow, and said plunger including means for forcing air through the upper opening and said air flow exiting out the lower opening of said chamber, and a spring mechanism forcing the plunger to its original position when released and actuating air flow into said chamber prior to a subsequent cycle.
 5. The device recited in claim 1 including a chamber for said thermistor, the size of said chamber and said thermistor being determined as a function of the desired wave shape of the output signal for said thermistor. 